U.S. patent number 4,713,311 [Application Number 06/559,178] was granted by the patent office on 1987-12-15 for homogeneous photoconductive layer of amorphous silicon and hydrogen.
This patent grant is currently assigned to Licentia Patent-Verwaltungs-GmbH. Invention is credited to Karl-Heinrich Greeb, Roland Herkert, Norbert Marschall, Wilhelm Senske.
United States Patent |
4,713,311 |
Senske , et al. |
December 15, 1987 |
Homogeneous photoconductive layer of amorphous silicon and
hydrogen
Abstract
The present invention relates to an electrophotographic
recording material and to a process for manufacturing it. The
material comprises an electrically conductive substrate and a
photoconductive layer of amorphous silicon and hydrogen applied
thereto. The recording material is characterized by only a single
photoconductive layer having an oxygen component of about 1 ppm to
1 atom percent. The recording material is produced by means of
cathode sputtering.
Inventors: |
Senske; Wilhelm (Eschborn,
DE), Herkert; Roland (Frankfurt, DE),
Marschall; Norbert (Frankfurt, DE), Greeb;
Karl-Heinrich (Dreieich, DE) |
Assignee: |
Licentia
Patent-Verwaltungs-GmbH (Frankfurt am Main, DE)
|
Family
ID: |
6180165 |
Appl.
No.: |
06/559,178 |
Filed: |
December 7, 1983 |
Foreign Application Priority Data
Current U.S.
Class: |
430/127;
430/65 |
Current CPC
Class: |
G03G
5/08221 (20130101) |
Current International
Class: |
G03G
5/082 (20060101); G03G 005/082 () |
Field of
Search: |
;430/65,66,84,95,127,130 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
4409311 |
October 1983 |
Kawamura et al. |
4414319 |
November 1983 |
Shirai et al. |
4461820 |
July 1984 |
Shirai et al. |
4489149 |
December 1984 |
Kawamura et al. |
|
Primary Examiner: Goodrow; John L.
Attorney, Agent or Firm: Spencer & Frank
Claims
What is claimed is:
1. A process for producing an electrophotographic recording
material having an electrically conductive substrate and an
homogeneous photoconductive layer of amorphous silicon and hydrogen
applied thereon, said process comprising
precipitating a photoconductive layer of silicon onto a substrate
maintained at a temperature ranging from 100.degree. to 300.degree.
C. by cathode sputtering in an atmosphere consisting of oxygen or
an oxygen-releasing gas, hydrogen and argon, wherein the oxygen or
oxygen-releasing gas is present in a proportion of about 1 ppm to 1
vol.% to said hydrogen and said argon gases.
2. The process of claim 1, further comprising
precipitating a structure-determining layer of silicon on the
substrate prior to depositing the photoconductive layer, said
structure-determining layer having a thickness of about 0.1 nm to
100 nm and being deposited in an atmosphere containing hydrogen,
argon and oxygen gases, said oxygen gas being added to said
hydrogen and said argon gases in an amount of about 1 ppm to 50
vol.%.
3. The process of claim 1, further comprising
precipitating a structure-determining layer of silicon on the
substrate prior to depositing the photoconductive layer, said
structure-determining layer having a thickness of about 0.1 nm to
100 nm and being deposited in an atmosphere containing hydrogen,
argon and a carbon- or nitrogen-releasing gas, said carbon- or
nitrogen-releasing gas being added to said hydrogen and said argon
gases in a proportion of about 0.1 ppm to 50 vol.%.
4. The process of claim 1, wherein
the proportion of said oxygen gas in the atmosphere is maintained
constant throughout the process.
5. The process of claim 1, wherein
said silicon layer contains oxygen atoms which are homogeneously
distributed throughout the thickness of the silicon layer.
6. The process of claim 1, wherein
said silicon layer is the sole photoconductive layer of the
electrophotographic recording material.
7. The process of claim 1, wherein said silicon is amorphous
silicon.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an electrophotographic recording
material which contains an electrically conductive substrate and a
photoconductive layer of amorphous silicon and hydrogen applied to
the substrate, and to a method for producing such an
electrophotographic recording material.
Amorphous silicon layers for indirect electrophotography have a
mechanical hardness and heat resistance which is much greater than
that of prior art record carriers. At the same time, such layers
exhibit sensitivity over a broad spectrum and a level of
sensitivity which, in almost the entire visible range, lies above
that of materials presently employed in practice. The use of
amorphous silicon leads to a significant improvement in copying
machines with respect to the service life of the photoconductors
and the copying speed. Moreover, amorphous silicon is a nontoxic
material and thus excellently environmentally compatible.
The production of electrophotographic layers of amorphous silicon
can be effected by means of two methods. In the past, production by
means of a silane glow discharge has been used most frequently. In
this process, the silane gas, monosilane or higher order silane, is
decomposed in a high frequency plasma discharge, with an amorphous
silicon hydrogen alloy being precipitated on heated substrates. The
hydrogen is necessary for the realization of good electrical and
optical characteristics. In order to produce
electrophotographically suitable layers, it is known to add a small
amount of diborane to the silane atmosphere, and possibly also
oxygen. See DE-OS No. 3,117,035. This manufacturing process
requires the use of highly toxic, easily flammable gases and gas
mixtures. The toxicity of diborane, in particular, is expressed in
its very low maximum workplace concentration value of 0.1 ppm.
Although the resulting layers are no danger to health, since they
are hard solid state layers, extensive and thus costly measures
must be taken during the manufacturing process for handling the
above-mentioned gases as well as for removing the gas mixtures
discharged from the coating apparatus.
One process which does not require the use of health endangering
gases, such as silane or diborane, is the cathode sputtering
process. In this process, the ionized gas atoms from a noble gas
plasma discharge, generally employing argon, eject particles from a
solid silicon target (cathode) which precipitate in a layer on the
heated substrate. The hydrogen required to realize suitable
properties is mixed in with the noble gas, with the sputtering, in
contradistinction to the silane glow discharge process, permitting
a variation in the hydrogen content and thus in the properties of
the resulting coatings. See T. D. Moustakas, J. Electr. Mater. 8,
391/1979.
It is known that electrophotographic record carriers of amorphous
silicon can be produced with the sputtering process. However, only
in multiple layer arrangements and by varying the hydrogen
component as well as by including SiO bonds in one of the partial
layers, as disclosed in European Pat. No. 0045204, and possibly by
subsequently coating the record carrier with a covering layer will
such record carriers exhibit electrophotographic usefulness.
Both manufacturing processes, sputtering and glow discharge, differ
principally in the composition of their gases and in the kinetic
energies of the gas molecules, which are determined by the gas
pressure. Therefore, the coatings produced with these two processes
also differ noticeably in their solid state characteristics, such
as, for example, charge carrier mobility or hydrogen inclusion.
Moreover, it cannot be expected that, for example, the doping
properties in both processes are the same.
Although it is known that in the silane glow discharge process,
high specific resistances can be realized by adding oxygen, (E.
Holzkampfer, J. Stuke, R. Fischer, 4th Photovoltaic Solar Energy
Conference, Stresa (1982), Ed. W. H. Ploss, G. Grassi, D. Reidel
Publ. Comp.), with large amounts of oxygen the spectral sensitivity
range shifts in a disadvantageous manner, due to the formation of
SiO.sub.x, to much shorter wavelengths. With smaller amounts of
added oxygen in the glow discharge process, high resistances can be
realized only by adding diborane. See DE-OS No. 3,117,035. Large
amounts of oxygen in the sputtering atmosphere, however, and thus
proportions greater than 1 at% in the coatings, result in a
decrease in resistance and thus in coatings which are unusable for
electrophotographic purposes See, B. G. Yacobi, R. W. Collins, G.
Moddel, P. Viktorovitch and W. Paul, Phys. Rev. B 24, 5907 (1981).
Here it becomes evident how different the effects are of the same
amounts of added gas in both processes. Without the addition of
oxygen, only dark resistances in the range from 10.sup.10 to
10.sup.11 ohm cm could be realized by sputtering of amorphous
silicon, values much too low for electrophotographic purposes. See,
W. Paul in F. Yonezawa (Ed.), Fundamental Physics of Amorphous
Semiconductors, Springer Series in Solid State Sciences, Volume 25
(1981), page 72 et seq.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an
electrophotographic recording material of amorphous silicon which
meets a number of requirements in the same manner. More
particularly, it is an object of the present invention to provide
such a recording material in which the electrophotographic
photoconductor layer comprises a single, homogeneous layer, and in
which no handling of dangerous, toxic material is necessary during
manufacture. Moreover, the number of doping gases to be added is to
be as low as possible. In particular, the use of compensation
dopings for which handling is critical is to be avoided.
A further object of the present invention is to provide a method
for manufacturing such an electrophotographic recording
material.
Additional objects and advantages of the present invention will be
set forth in part in the description which follows and in part will
be obvious from the description or can be learned by practice of
the invention. The objects and advantages are achieved by means of
the processes, products, instrumentalities and combinations
particularly pointed out in the appended claims.
To achieve the foregoing objects and in accordance with its
purpose, the present invention provides an electrophotographic
recording material comprising an electrically conductive substrate
and a photoconductive layer of amorphous silicon and hydrogen
applied to the substrate, wherein the material contains a single
photoconductive layer having an oxygen concentration of about 1 ppm
to 1 atom percent as the sole photoconductive layer in the
electrophotographic recording materials.
Preferably, a material which determines the structure of the
photoconductive layer is applied between the substrate and the
photoconductive layer. The thickness of the structure determining
layer preferably is between 0.1 and 100 nm. The structure
determining layer preferably is made of SiO.sub.x
(0.1.ltoreq..times..ltoreq.2), SiC.sub.y (0.1.ltoreq.y.ltoreq.1) or
SiN.sub.z (0.1.ltoreq.z.ltoreq.1.3).
In one preferred embodiment of the invention, an intermediate layer
is disposed between the substrate and the photoconductive layer for
blocking the carrier injection.
The present invention also provides a process for producing an
electrophotographic recording material having an electrically
conductive substrate and a photoconductive layer of amorphous
silicon and hydrogen applied to the substrate, wherein the cathode
sputtering process having a sputtering atmosphere of argon and
hydrogen is employed to apply the photoconductive layer of
amorphous silicon and hydrogen as the sole photoconductive layer in
the electrophotographic recording material, and a proportion of
about 1 ppm to 1 volume percent oxygen or oxygen releasing gas are
added to the sputtering atmosphere of argon and hydrogen to provide
an oxygen concentration in the range of about 1 ppm to 1 atom
percent in the photoconductive layer.
Preferably, an oxygen component of about 1 ppm to 50 volume percent
is added only at the beginning of the growth process until a
structure determining layer thickness of about 0.1 to 100 nm is
reached.
In one embodiment of a process for producing an electrophotographic
recording material having an electrically conductive substrate and
a photoconductive layer of amorphous silicon and hydrogen applied
to the substrate, the cathode sputtering process having a
sputtering atmosphere of argon and hydrogen is employed to apply
the photoconductive layer of amorphous silicon and hydrogen as the
sole photoconductive layer in the electrophotographic recording
material, the sole photoconductive layer having an oxygen
concentration of about 1 ppm to 1 atom percent, and for the
structure determining layer a proportion of about 1 ppm to 50
volume percent nitrogen releasing and/or carbon releasing gas is
added to the sputtering atmosphere of argon and hydrogen at the
beginning of the growth process until a layer thickness of about
0.1 to 100 nm is reached.
It is to be understood that both the foregoing general description
and the following detailed description are exemplary and
explanatory, but are not restrictive of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Surprisingly, it has been found that it is possible in the
sputtering process, without the use of compensatingly acting doping
gases and with the addition of only the lowest oxygen
concentrations, to realize resistances far above 10.sup.12 Ohm cm
and thus obtain homogeneous amorphous silicon layers which are
suitable for electrophotographic purposes. The oxygen addition
range lies between about 1 ppm and a maximum of 1 atom percent
preferably between 1 and 100 ppm. Such small quantities of O.sub.2
are sufficient in the sputtering process to greatly reduce the dark
conductivity in that presumably the oxygen atoms contribute to the
saturation of free valences of the Si atoms and thus reduce the
electronic state density in the band gap. At the same time, the
addition of oxygen in the ppm range presumably creates traps which
reduce charge transport in the dark. Trap concentrations of at
least 10.sup.16 to 10.sup.17 cm.sup.-3 are necessary for the
electrophotographic chargeability of a material so as to reduce the
charge carrier injection on the part of the substrate even without
a blocking layer. Such trap concentrations can be developed by
accurately setting the manufacturing conditions, such as the
argon/hydrogen ratio between 1/0.03 and 1/0.5, substrate
temperature between 100.degree. and 300.degree. C., flow rate
between 0.2 and 100 standard cubic centimeters per minute and an
addition of oxygen in the amount of a few ppm. However, the trap
concentration must not be too high, that is at maximum about
10.sup.19 cm.sup.-3, since otherwise the mobility of the charge
carriers and thus the photosensitivity of the layers becomes
uselessly low. This is what determines the upper limit of the
amount of oxygen to be added. The same effect as realized from the
addition of oxygen can be realized by the addition of gases which
release oxygen, for example laughing gas, N.sub.2 O for the
photoconductive layer.
Examples for gases that release nitrogen or carbon for the
fabrication of the structure determining layer are nitrogen
trihydride (NH.sub.3) or methane (CH.sub.4).
The advantages realized by the present invention are, in
particular, that the amorphous silicon can be produced for use in
electrophotography in a process which does not require the use of
any toxic or self-combustive gases. This permits the omission of
complicated and at the same time cost intensive measures for the
handling of toxic gases and for the elimination of gases discharged
by the pumps. The image carriers may be applied as homogeneous
layers, with no blocking layer being required. No gases other than
hydrogen and oxygen need be added. A compensation for residual
conductivity by the addition of diborane can be omitted. Record
carriers produced in this manner exhibit charge field intensities
of more than 40 V/.mu.m. The sputtering process, operated with high
frequency or direct voltage, permits high growth rates,
particularly when using a magnetron sputtering process. Thus, this
process offers advantageous conditions for use in industrial
production.
In a preferred embodiment of the present invention, an intermediate
layer of, for example, SiO.sub.2, Al.sub.2 O.sub.3, CeO.sub.2, is
introduced between the photoconductor and the substrate to provide
a better injection blockage. The thickness of the blocking layer is
suitably selected to be between 5 and 500 nm. This makes it
possible to increase the charging limit to more than 60 V/.mu.m
with a positive charge and a simultaneous reduction of dark
discharges.
A layer which determines the structure of the subsequently applied
photoconductive layer may also be applied to the substrate, if
required. In this way it is possible to produce a high trap
concentration, particularly at the interface with the substrate, to
prevent the injection of charge charriers into the photoconductive
layer due to the build-up of stationary space discharges. The
function of structure determining layers cannot be compared with
that of a blocking layer. Blocking layers are active elements which
serve the purpose of preventing the injection of charge carriers
from the substrate into the photoconductor. To be able to perform
such a function, they must have a sufficient thickness so that the
charges accumulating at the interfaces are prevented from passing
through even in small quantities. Structure determining layers,
e.g. those of SiO.sub.x, SiC.sub.x or SiN.sub.x, however, may be
effective already in thicknesses of a few nm since they are
intended merely to produce a bond configuration which results in
high trap concentrations in the photoconductive layer.
The following examples are given by way of illustration to further
explain the principles of the invention. These examples are merely
illustrative and are not to be understood as limiting the scope and
underlying principles of the invention in any way.
EXAMPLE 1 FOR AN ELECTROPHOTOGRAPHIC RECORDING MATERIAL
A photoconductive amorphous silicon layer is produced at a
substrate temperature of 250.degree. C. with a power density of
about 1 W/cm.sup.2 in argon with 30 Vol% hydrogen, a gas
flowthrough of 20 standard cubic centimeters per minute, a pressure
of 10 mTorr and an oxygen concentration of less than 10 ppm. This
layer has a charge level of 44 V/.mu.m.
EXAMPLE 2
A layer combination with blocking layer, with an amorphous silicon
photoconductor layer having the above-mentioned data of Example 1
is employed. An Al.sub.2 O.sub.3 layer of a thickness of 1000 .ANG.
is applied as an injection blocking layer on the conductive
substrate. The layer combination has a charge level of 74 V/.mu.m.
With a photoconductive layer thickness of about 2.5 .mu.m, the
electrophotographically measured photosensitivity of this
photoconductive layer, between 400 and 500 nm, reaches the maximum
possible photoelectric gain of 1, and by increasing the layer
thickness, the high spectral sensitivity can be expanded to the
entire visible range.
It will be understood that the above description of the present
invention is susceptible to various modifications, changes and
adaptations, and the same are intended to be comprehended within
the meaning and range of equivalents of the appended claims.
* * * * *